It is when a country attacks another country (or entity) that it fears may plan to attack it later. In other words, it takes action not because of factual threat but of mere potential risk. The precautionary principle is also applied when someone is detained or jailed not because of what he has actually done but because of what the government or police think he may potentially do.
The United States attacked and occupied Iraq using the Precautionary Principle. Nothing would prevent it to use the same principle to attack Iran.